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Typical steps in doing a ChE design case study using HYSYS or UniSim

 

It is assumed that the starting point is a detailed base case as given by Turton et al., with a pfd and specifications for major equipment, process streams and utilities.  Note that such specifications generally contain deliberate errors, which must be corrected while doing the base case.  There will also be inefficient ways of doing things, and sometimes even impossibilities, which will provide opportunities for making improvements in your design.  Throughout, look at the helpful advice at http://people.clarkson.edu/~wilcox/Design/refhysys.htm.

 

I.      Base case

  1. Level-1 simulation.  Uses provided information with simplified heat exchangers, reactors, separators etc.  The aim is material and heat balances, without performing detailed calculations for equipment.  Detailed calculations will be required at Level 2 in order to size the equipment.  Your base case equipment sizing and utility requirements will differ from those in the provided base case and are necessary to do a cost estimation.

1.      Enter the components in the process streams (not the utilities, such as steam) into the Components section of the Simulation Basis Manager.

2.      Select an appropriate thermodynamics package using, for example, the methods shown at http://people.clarkson.edu/~wilcox/Design/thermodl.htm .  Enter this into the Fluid Package Basis.

3.      From the provided base-case component flow rates, calculate the conversions for the reactions and enter these into the Reactions section of the Simulation Basis Manager.

4.      Go to the Simulation environment and enter the feed streams on the left of the pfd.

5.      Proceed with entering other units as given in the base case pfd.  Try to use the conditions given in the base case even though these may turn out later to be impossible at Level 2.  If you need numbers for streams that have not been numbered in the provided base case, use numbers such as 2a or 3b that are located right after 2 and 3.

6.      For heat exchangers use heaters and coolers.  Only duties are calculated via heat balances, and utility requirements are left to Level 2.

7.      Use conversion reactors, with the conversions entered in part 3 above.

8.      Use component splitters for all separators.  Set the temperatures and pressures of the streams to those provided.   If no temperature is given for a stream exiting a stripping or absorption column, set the pressure to that provided for the unit and the heat flow to 0 (adiabatic).  HYSYS/UniSim should then calculate the temperature.  It is likely that you will encounter discrepancies that must be resolved in your Level 2 Simulation, such as temperatures or pressures that don’t make sense, or overhead vapor products that have some liquid in them.  Note that a splitter representing a distillation column includes recycle and reboiler loops, so these units should not be added to the pfd. 

9.      Click on the Workbook icon above the pfd, then Workbook, Setup.  For Materials Streams click on Add on the right side and select Component Molar Flow, then the All radio button.  Format to 4 significant figures.  In the upper right side of the Setup menu click on Order, and then select Ascending.  The final result should be a table something like that given in the provided base case.  If you print your Workbook formatted in this way, you can quickly check that everything agrees with the provided base case.

10.  While doing all this, keep notes on things that may be wrong or non-optimal for use later.

11.  When you have a converged pfd with the conditions roughly as specified, print out the pfd and the Workbook for comparison to the provided base case.

 

  1. Level 2 simulation.  Correct obvious errors and have HYSYS/UniSim do design calculations for “real” equipment.  From these calculations you will get the equipment and utility specifications needed for cost estimation.  Your results will differ from the provided base case, while using the same (or nearly the same) basic pfd and feed streams.

1.      Save your case study under a different name so you don’t lose your Level 1 simulation.

2.      Open the Basis Manager.  Enter the provided reactions kinetics into the Reactions section of the Basis Manager.  When done, click on “Add to FP.”

3.      You now need to create one or more additional Fluid Packages for the utility streams.  Following are instructions for cooling water and heating steam required by heat exchangers.   On Components click on Add, select water, and click x.  You should now have a new Component List with only water in it.  Click on Fluid pkgs, Add, then select ASME Steam.  You should now have a Basis-1 fluid package for the process streams and a Basis-2 fluid package with water-steam.  Note that you can add additional fluid packages for specific units if it happens that the same thermodynamic model is not suitable for all conditions in your plant.  HYSYS/UniSim does allow this.

4.      Return to the Simulation Environment.  Delete the first conversion reactor and substitute a “real” reactor that requires kinetics (plug-flow or stirred-tank).   Since HYSYS/UniSim will recalculate the entire pfd every time you make a change, the process of inserting a reactor and adjusting the conditions can be very time consuming and may result in problems elsewhere in the pfd.  If this happens, try optimizing your reactor in a separate case using the same reactor feed conditions.

5.      Delete the first separator and substitute a “real” piece of equipment, e.g. an absorber or distillation column.  For distillation, absorbing or stripping, first arrange (on paper) all components in order of increasing boiling point (which you can get from a handbook, or from the View Component feature of the Basis Manager.  For the desired separation, select the heavy and light key components, i.e. the split where you want lower boiling components above the light key to go almost entirely overhead and the higher boiling components below the heavy key to go almost entirely out in the bottoms.  You may want to use the “Short Cut Distillation” unit first to do approximate calculations that will give you an idea of what the number of trays, reflux ratio, and feed tray should be.  Again, you may first want to experiment with the separator alone in a separate case to avoid long computational times or problems with recycle.

6.      Once you have inserted all real reactors and separators into your new pfd, you can begin substituting real heat exchangers for heaters and coolers.  This can be done in a separate case.  There’s no need to insert these into the main case at all.  Use the method shown at http://people.clarkson.edu/~wilcox/Design/hxsizing.htm to get the areas of each heat exchanger.

7.      Determine the detailed information on each column using the methods at http://people.clarkson.edu/~wilcox/Design/traysize.htm and the reboiler and condenser sizes using http://people.clarkson.edu/~wilcox/Design/condsize.htm.

8.      Make sure you have included appropriate pressure drops everywhere using the advice at http://people.clarkson.edu/~wilcox/Design/Pressure.htm.

9.      Make sure you have included and sized all compressors and pumps required (see the page cited in 8 above). You’ll require a compressor or pump for each recycle stream, including column reflux.  Usually you will not require a pump between the column bottom and the reboiler because of the thermosiphon effect, i.e. the downflowing liquid has a much greater density than the upflowing gas causing an automatic pressure difference that drives the flow.  (“Thermosiphon” is sometimes spelled thermosyphon.  See page 11-13 in Perry's for a good explanation of thermosiphon reboilers.)  If the condenser is significantly above the top of the column, you would expect the same thing for a condenser, i.e. for the thermosiphon effect to provide the delta P required for the flow.  However, condensers are usually located on the ground for easy access for maintenance.  Thus a pump is required that is sufficient to provide the hydrostatic head from the bottom of the column to the top.  As an option, you can check on thermosiphon reboilers using, for example, http://www.distillationgroup.com/technical/016__abs.htm.

10.  Estimate the costs of all utilities and equipment using the methods in the textbook and at http://people.clarkson.edu/~wilcox/Design/refcosts.htm.  Use these costs to estimate the NPV and DCFRR using the methods shown in the text and the specified tax rate, required rate of return, depreciation method, etc.

 

II.   Your improved case

Make improvements to the base case so as to increase the NPV and DCFRR while meeting legal requirements and ethical concerns for safety and the environment.  This will involve insertion and/or removal of units and changing operating conditions.  Examples of improvements are modifying the reactor temperature, pressure and dimensions; substituting a different feed stream (such as oxygen for air); modifying the reflux ratio or splits in a distillation column; adding a recycle stream; replacing a valve with an expander when the pressure of a gas stream must be dramatically reduced; avoiding increasing the pressure of a stream and then reducing it; cooling and condensing a vapor stream so that a pump can replace a compressor to increase pressure.  Make certain the production rate is as specified, the purity of the product meets typical customer requirements, and that the temperature, pressure and composition of a given feed stream is not changed (although the flow rate will surely be different from the base case).   Keep a record of the things you try in a table.

 

III.  Final report.

Prepare the final report in accordance with the instructions at http://people.clarkson.edu/~wilcox/Design/casetips.htm.

 

Last revised July 27, 2006.  Please submit all questions, comments and suggestions to W.R. Wilcox

 

Disclaimer: The material on these pages is intended for instructional purposes by Clarkson University students only.  Neither Clarkson University nor Professor Wilcox is responsible for problems caused by using this information.

 

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